The continuous casting practice of a molten metal most commonly adopted at present with a continuous casting machine comprises: forming a cast strand by teeming a molten metal, received from a ladle into a tundish, into a mold arranged below a pouring nozzle through said pouring nozzle attached to the bottom wall of the tundish in the form of a downward projection; and withdrawing the cast strand thus formed into a long strand from the lower end of the mold substantially vertically downwardly then horizontally after a curve, while cooling the cast strand. After the completion of casting of the molten metal in the tundish, molten metal and slag remaining in the tundish are discharged and new molten metal is received in the tundish to start the next continuous casting.
The aforementioned discharge of molten metal and slag remaining in the tundish at the end of casting of molten metal is conventionally carried out by discharging the remaining molten metal and slag over the upper edge of the side wall of the tundish while tilting the tundish, or by discharging same through the pouring nozzle.
However, tilting of the tundish requires a large-sized tilting mechanism, which in turn not only requires large installation costs but also leads to a complicated equipment arrangement because of the combination of the tilting mechanism and the other devices of the continuous casting machine. The discharge of molten metal and slag remaining in the tundish through the pouring nozzle results in the pouring nozzle being seriously eroded by the slag. Recently, in particular, an expensive sliding gate comprising refractories is often provided between the bottom wall of the tundish and the pouring nozzle with a view to starting and discontinuing casting of molten metal in the tundish and adjusting the flow rate of the molten metal, and in this case, a problem encountered is that, when discharging molten metal and slag remaining in the tundish through the sliding gate and the pouring nozzle, not only the pouring nozzle but also the sliding gate is seriously eroded by the slag, thus shortening their service life and increasing the operating cost of the tundish.
The above-mentioned conventional continuous casting machine (hereinafter referred to as the "vertical type continuous casting machine") is of the type in which molten metal in the tundish is substantially vertically downwardly cast. Accordingly, the formed cast strand, withdrawn vertically for most of the casting process, although eventually bent horizontally, requires a very large height of the equipment, resulting in a vast investment in construction of the installation including the building. To overcome this inconvenience, a horizontal type continuous casting machine has just been developed principally in an attempt to reduce the construction cost, which comprises: horizontally attaching a pouring nozzle to the lowermost portion of the side wall of a tundish; arranging a mold horizontally on the same axis as the horizontal axis of said pouring nozzle, in close contact with the tip of said pouring nozzle; forming a cast strand by horizontally teeming a molten metal, received into the tundish, into the mold through the pouring nozzle; and withdrawing the cast strand thus formed always horizontally into a long strand while cooling.
Even in the aforementioned horizontal type continuous casting machine, the discharge of molten metal and slag remaining in the tundish at the end of casting of molten metal should, as in the conventional vertical type continuous casting machine, be carried out either by tilting the tundish or by using the pouring nozzle. There are therefore problems similar to those in the conventional vertical type continuous casting machine, such as the necessity of installing a tilting mechanism and the erosion of the pouring nozzle by slag.
Furthermore, in the horizontal type continuous casting machine, it is very difficult, in terms of technology as well as of equipment, to provide a sliding gate for starting and discontinuing casting of molten metal and adjusting the flow rate of the molten metal in the narrow space between the side wall of the tundish and the pouring nozzle. As far as we know, in fact, there is as yet no tundish for a horizontal type continuous casting machine, equipped with such a gate. Therefore, when a breakout accident of molten metal is caused by a breakage of the shell of a non-solidified cast strand downstream the exit of the mold in the conventional vertical type continuous casting machine, the breakout of molten metal can be quickly prevented by closing the gate, thus minimizing the damage caused thereby. In contrast, when such a breakout accident is caused in the horizontal type continuous casting machine, molten metal in the tundish totally flows out through the pouring nozzle and the mold because of the absence of such a gate. In addition, since a water collection channel for cooling water is arranged below the mold exit, molten metal flowing into this channel may cause a serious accident such as a steam explosion.
In the horizontal continuous casting machine, when the surface of molten metal in the tundish becomes lower than the highest point of the inner surface of the mold toward the end of casting of molten metal, a cast strand having a shape matching with the inside diameter of the mold cannot be obtained, resulting in the formation of a portion with a defective shape in the trailing end of a cast strand with a normal shape. This leads to trouble in withdrawing and hot shearing a cast strand.
With these facts in view, development is strongly demanded of a discharging mechanism for molten metal and slag remaining in a tundish for a continuous casting machine, which is capable of easily, certainly, safely and rapidly discharging molten metal and slag remaining in the tundish at or toward the end of casting of molten metal or at the occurrence of a breakout accident of molten metal caused by a breakage of the shell of a non-solidified cast strand. No such discharging mechanism has as yet been proposed.